The present invention relates to a system and apparatus for coulometrically determining the thicknesses of layers of metallic coatings.
At present, there are known and commercially used instruments for measuring coating thicknesses using the coulometric principles based on Faraday's law. The coating thickness is determined by measuring the total quantity of electricity which is necessary for the anodic dissolution of the coating. To do so, the instruments generally employ a cell containing a supply of suitable electrolyte, a power supply necessary for the electro-chemical dissolution of the coating, an indicator of the start and completion of the dissolution of the coating, a meter for evaluation of the duration of dissolution whereby the thickness is measured, and means for automatically controlling the entire operation. However, the need to guarantee constant electrochemical condition during the course of the measurement, which is a fundamental requisite for desirable repeatability and accuracy of the apparatus leaves much to be desired. In order to obtain repetitive accuracy and reproducibility of the thickness measurement, a cell must be provided which exactly limits the surface area at which the dissolution of the coating takes place, and furthermore insures the supply of the proper electrolyte to the surface. It is this cell which, in the known systems, has numerous disadvantages.
In several of the known devices, the electrolytic cell comprises an open vessel which is filled manually with the electrolyte, using for example, an "eye" dropper, pipette, syringe or the like. During the measuring period, the electrolyte in the vessel is usually agitated in an attempt to insure proper mixture. Cells of this type require a relatively large working area, for example, a diameter equal to 1.5 to 3.5 mm. Such apparatus do not allow measurements on smaller areas, or a dissolution faster than about 20 to 50 .mu.m per minute.
Another known type of cell is equipped with a separate electrolyte reservoir. The electrolyte is fed from the reservoir to the surface area being measured by a pump in an attempt to ensure the exchange of electrolyte at the site being measured. This type of apparatus allows a reduction in the work area being measured to a diameter of 1 mm, but is somewhat complex and difficult to manage, and often results in losses of electrolyte.
Reference can be made to the following showing the relevant prior art:
Kutzelnigg A.: Die Prufung metallischer Uberzuge, (S.71) E. Leuze Verlag, Saulgau, BRD, 1965 PA1 Plog H.: Schichtdickenmessung, (S.19), E. Leuze Verlag, Saulgau, BRD, 1967 PA1 Biestek T., Sekowski S.: Methoden zur Prufung metallischer Uberzuge (S.100), E. Leuze Verlag, Saulgau, BRD, 1973
Furthermore, experience with the operation of these types of cells confirms that the electrolyte transfer, at the beginning and at the end of the measuring period, causes difficulties when placing the cell on, or removing the cell from the workpiece being measured, because the electrolyte tends to escape from the cell most easily at these times. In addition, an effective electrolyte exchange is not fully assured during operation since a partial blocking of the area being tested might occur through the formation of an air bubble in the cell. As a result, extensive variations in measurement accuracy and reproducibility may be encountered in practice.
It is the object of the present invention to provide a process and apparatus for producing the electrolyte exchange directly in the vicinity of a defined controlled are so as to significantly increase the measuring accuracy, to provide uniform coating dissolution over the area to be measured, reduce the size of the measured area to a value smaller than those presently used, and to increase the measuring speed.
It is an object of the present invention to provide a cell in the form of a probe which can be manually held and which contains the electrolyte, which insures proper exchange of electrolyte, and reliable definition of the situs.
It is the object of the present invention to provide a probe which will eliminate errors caused by ion depletion and improper mix at the surface of measurement. As a result of this, it is possible to reduce the size of the measurement area, decrease the time required for a measurement, and reuse the same electrolyte for successive measurements.
It is another object to provide a probe which is capable of holding the electrolyte, without loss of liquid, while the probe is removed from the test specimen. As a result, the probe now becomes truly portable, which means that the probe with its electrolyte can be moved, without emptying, from one surface area to another, or from one specimen to another.
It is another object to provide a system wherein the probe can be easily filled, emptied, or rinsed, without the use of auxiliary devices, such as eyedroppers or pipettes.
It is another object to provide a system by which measurements of multi-layer coatings requiring different electrolytes can be done without the use of auxiliary devices.
It is another object to provide a probe system composed to two main components; an outer sleeve, and a removable inner electrolyte storage chamber. This inner chamber can be removed and replaced with another chamber holding a different electrolyte, without disturbing the position of the outer sleeve which is in contact with the test specimen.
It is another object of the present invention to provide a probe which can be used on areas which are substantially inclined from the horizontal.
These objects as well as others together with numerous other advantages, will be apparent from the folowing disclosure.